1.1.5. Coulomb law in reality the principle of superposition The concept of density of charge: line density 2(r), surface density o(r) and volume density p(r") Point to line F ea ri(r 4丌 F=9 Point to surface a P(7) Point to volume 4兀S Line to line F A()(r) Br-r)dld - Surface to surface F o(ro(r 4T 8 r-r P(rp(r Volume to volume r-r)dsds 4兀S。1.1.5. Coulomb law in reality & the principle of superposition Point to line: Point to surface Point to volume Line to line Surface to surface Volume to volume The concept of density of charge : line density , surface density (r ) and volume density .   (r )    (r )   ds Q r F r r o o s a    2 ( ) 4     = dv Q r F r r o o v a    2 ( ) 4     = r dl r Q r F l a 0 2 0 ( ) 4     =    ' ( ) ( ) 4 1 3 ( ) ' dsds r r F r r o s s r r −  −     =            ' ( ) ( ) 4 1 3 ( ) ' dsds r r F r r o v v r r −  −     =            ' ( ) ( ) 4 1 3 ( ) ' dldl r r F r r o l l r r −  −     =           